Notes:

UML also needs to be ported, both to Linux on non-i386 and to other operating systems.
There's a ppc port underway, but not moving too quickly. It gets to the point of trying to run init. Paul Mackerras has offered to work on it, which is very cool.
The IA64 port is in a similar state, but not as advanced.
A Windows port was started, and it dies when trying to start kernel threads. A number of people have offered to help on this, so it might start moving soon. A structural problem with this port is that Windows effectively has 64K pages, since it only allows mmapping on 64K boundaries. This is not a problem for most apps, but those that use mmap in unusual ways won't run under UML/Windows without being recompiled for a 64K page size.